PBFA Z: A 55 TW/4.5 MJ ELECTRICAL GENERATOR* R. B. Spielman, W. A. Stygar, K. W. Struve, J. F. Seamen Sandia National Laboratories, Albuquerque, NM 87185-1194

Abstract The electrical energy is transmitted through a water- vacuum insulator interface and conducted via self PBFA Z is a new 55 TW/4.5 MJ short pulse electrical magntically-insulated vacuum transmission lines (MITLs) driver located at Sandia National Laboratories. We use to the z-pinch load. PBFA Z delivered a peak current of PBFA Z to magnetically-implode plasma shells. These 18 MA to a low inductance z-pinch load. The load, a configurations are historically known as z pinches. The tungsten wire array, generated a peak x-ray power of pulsed power design of PBFA Z[1] is based on 200 TW and an x-ray yield of 1.85 MJ. conventional single-pulse Marx generator, water-line The overall electrical design of PBFA Z pushed the pulse-forming technology used on the earlier Saturn[2] limits of pulsed power engineering. The physics of and PBFA II[3] accelerators. PBFA Z stores 11.4 MJ in MITLs and vacuum convolutes is still not well its 36 Marx generators, couples 4.5 MJ in a understood. Issues such as radiation-driven vacuum gap 55-TW/105-ns pulse to the output water transmission closure and limits on current density near the load need to lines, and delivers up to 3.0 MJ and 40 TW of electrical be studied in detail in the future. PBFA Z is studying energy to the z-pinch load. Depending on the initial load some of these fundamental pulsed power concerns while inductance and the implosion time, we attain peak addressing the z-pinch physics necessary for the next currents of 16-20 MA with a rise time of 105 ns. generation of drivers. Current is fed to the z-pinch load through self magnetically-insulated transmission lines (MITLs). Peak 2 PULSED POWER DESIGN electric fields in the MITLs exceed 2 MV/cm. The current As described in detail in Ref. 1, PBFA Z’s pulsed from the four independent conical disk MITLs is power design is based on Marx generator/water pulse- combined together in a double post-hole vacuum forming technology. PBFA Z contains 36 nearly identical convolute with an efficiency greater than 95%. The modules. In each module a Marx generator, with 60, measured system performance of the water transmission 1.3-µF charged to a voltage of 90 kV, delivers lines, the vacuum insulator stack, the MITLs, and the its energy to a water-dielectric coaxial in 1 µs. double post-hole vacuum convolute differed from preshot The capacitor reaches a peak voltage of 5 MV. A low- predictions by ~ 5%. Using a 2-cm radius and a 2-cm jitter -triggered gas switch is used to couple the µ length tungsten wire array with 240, 7.5- m diameter energy into a second, lower inductance coaxial water wires (4.1-mg mass) as the z-pinch load, we achieved x- capacitor in 200 ns. Self-breaking water switches are used ray powers of 200 TW and x-ray energies of 1.85 MJ as to transfer the energy into a 0.12-Ω constant impedance measured by x-ray diodes and resistive bolometry. water transmission line. The electrical pulse at this point has a voltage of 2.5 MV and a pulse width of 105-ns 1 INTRODUCTION FWHM. The total power generated in the accelerator at The PBFA-II accelerator[3] at Sandia National this point is 50 TW. The electrical energy is delivered to Laboratories was originally used by the light-ion-beam an insulator where is passes into the vacuum portion of ICF Program. We have made major modifications to the the accelerator. The electrical energy is then fed through accelerator in order to optimize coupling to magnetically- four vacuum MITLs, through a vacuum convolute, to the imploded loads, typically z pinches. The facility, in z-pinch load. A schematic of PBFA Z is shown in Figure operation since September 1996, has been renamed PBFA 1. Z. Much of its electrical design and design philosophy was descibed in detail in Ref. 1. 2.1 Water transmission Lines This paper describes the electrical performance of The water transmission lines used on PBFA Z are a bi- PBFA Z and summarizes the overall pulsed power design plate, constant-impedance, constant-anode/cathode-gap and modeling that lead to the successful operation of the design. This was done to optimize the energy efficiency of largest z-pinch driver in the world. To date PBFA Z has the transmission lines and to maximize the coupling of conducted more than 65 shots. We have demonstrated the the energy in the transmission lines to an inductive load. generation of greater than 50 TW of electrical power in The impedance of each of the 36 lines was 4.32 Ω. Each the constant impedance water tranmission lines in a water line has a voltage and current monitor. (See Section 2.5-MV, 100-ns FWHM voltage pulse. The measured 3.) We can accurately measure the voltage and current and, forward-going electrical energy is nearly 5 MJ. hence, the power and energy in each water transmission line. These measurements allow us to verify the operation of the water lines. These measurements are complicated

0-7803-4376-X/98/$10.00  1998 IEEE 1235 Figure 1 A schematic of PBFA Z showing the Marx generators, the water pulse forming section, the insulator stack, and the diagnostic line-of-sight. by the spatially- and temporally-varying behavior of the particle-in-cell (PIC) computer codes [1]. These MITLs electrical pulse and its reflection from the load. The must operate at fields 10X greater than the explosive transmission line gap is fixed at 14 cm. The peak voltage emission threshold on the cathode and must hold off the in the water lines varies with the proximity to the load full accelerator voltage for 100-150 ns. We place current due to the superposition of the reflected pulse on the main monitors at two radial locations and 3-6 azimuthal pulse. The water transmission lines were modeled with postions in each of the four MITLs to better study losses. electrostatic codes to examine the issues of enhanced and fringing electric fields. 2.2 Insulator Stack The insulator stack (See Figure 2.) is the boundry between the water dielectric and the vacuum that is necessary to drive the z-pinch load. The insulators, nearly 4 m in diameter, are made from Rexoliteª, a high-density cross-linked polystyrene. Experience has shown that Rexoliteª has superior mechanical and electrical properties when compared with PMMA or polycarbonate. The electric field was carefully “graded” across each insulator to equalize the stress on each insulator component. The insulator stack was designed with a 2-D electrostatic code and then the final design validated with a 2-D electromagnetic code. We expect that future, more advanced designs, will use electromagnetic design codes exclusively. The voltages and currents were measured at each of the insulator stacks. (See Section 3.) These data gave a very accurate measure of the power and energy Figure 2 A schematic of the PBFA-Z insulator stack and through the insulator stack boundary. MITLs is shown. Note, only half of a cylindrically- symmetric section is displayed. The diameter of the 2.3 MITLs insulator stack is nearly 4 m. One of the great achievements of PBFA Z is the design of the magnetically-insulated vacuum transmission lines 2.4 Vacuum Convolute (MITLs). The MITLs consist of four separate, conical The most critical part of the vacuum power flow is the disk feeds coupled together at the vacuum convolute. (See double post hole convolute that couples the four, conical Section 2.4.) These MITLS operate successfully at a peak disk MITLs together. This is done in order to combine the electric field of 2MV/cm with vacuum gaps as small as current from each level and deliver the summed currents to 1 cm. Issues such as vacuum electron flow and plasma a single z-pinch load. A vacuum convolute is necessarily formation were addressed with 2-D electromagnetic a three dimensional object and will necessarily have a

1236 number of magnetic field nulls. It is the electron losses voltage monitors and 36, magnetic loop current probes through these nulls that drive the convolute design. We (B-dot monitors). These final 72 monitors give us a based the PBFA Z design on the successful Saturn design. complete measure of the timing, voltage, current, power, In addition, the convolute was partially modeled using and energy in each of the 36 accelerator modules. The QUICKSILVER [1], a 3-D E&M PIC code. A drawing of voltage and the current at the insulator stack is again the convolute is given in Figure 3. measured with 6, V-dot probes and 3, B-dot probes on each of the four levels. Current in the MITLs is measured with up to six B-dot probes at two separate radial positions in each of the four MITLs. The total load current is monitored with up to four B-dot probes located about 5 cm from the z-pinch load.

3.0 60

2.5 50

2.0 40 Power (TW)

1.5 30

1.0 20 Voltage (MV)

0.5 10

0.0 0

-0.5 -10 2200 2300 2400 2500 2600 2700 2800

Time (ns) Figure 4 The average voltage (solid line) and total power waveforms in the water transmission lines measured on Shot 51.

6 3.5 Figure 3 A line drawing of the posthole convolute for PBFA Z. The drawing shows the anode and cathode 5 3.0 connections presently in use. 2.5 4

2.0 Voltage (MV) 3 ELECTRICAL DIAGNOSTICS 3 1.5 One of the most important parts of any successful 2 accelertor is the attention paid to electrical diagnostics. Current (MA) 1.0 1 This is particularly true of single shot, pulsed 0.5 accelerators. We field a full complement of diagnostics on 0 every shot. With thirty six modules, this translates into 0.0 hundreds of waveforms per shot. -1 -0.5 2350 2400 2450 2500 2550 2600

3.1 Diagnostic Description Time (ns) The operation of the Marx generators is monitored with Figure 5 The currents (solid lines) and voltages for each 36, current viewing resistors (CVRs) located at the ground level of the insulator stack measured on Shot 51. connection. The voltages on the water-dielectric intermediate store (IS) capacitors are measured with 3.2 Calibration capacitively-couple voltage monitors, commonly referred Calibration of all of the diagnostics is critical. Some of to as dV/dt or V-dot monitors. Again, there are 36 of the calibrations are done in-situ with pulsers and reference these V-dot monitors. The operation of the laser triggered monitors placed near the diagnostics under calibration gas switches is checked with these IS V-dot monitors. while other calibrations were done externally. In all cases The electrical output of the IS capacitors charges the the pulsers are designed to provide electrical test pulses second intermediate store capacitor (the Line 1 or L1 with nearly the same rise time as the actual accelerator capacitor). Again there are V-dot voltage monitors at this pulse. The reference monitors are carefully calibrated location. AS many as 36 monitors are used to measure against NIST-traceable standards. All calibration the voltage and switching of the L1 capacitors. The waveforms are stored and are available for examination or output of the L1 capacitors is monitors with 36, V-dot further checking at any time in the future. Typical

1237 electrical diagnostic calibrations were accurate to ~ 2-4%. shows the voltage and current waveforms measured on We believe that the total error, including data acquisition each of the four insulator stacks for PBFA-Z Shot 51. and systematic errors, for most of the electrical Figure 6 shows the total current through the insulator diagnostics is better than 5%. stack together with the electrical power (I*V) passing through the insulator stack. The peak electrical power 4 ELECTRICAL PERFORMANCE measured on Shot 51 was 46.8 TW. The measured This section of the paper describes the actual electrical energy at the insulator stack for Shot 51 was performance of PBFA Z element by element in the 2.9 MJ. accelerator. 20 4.1 Water Transmission Lines Measurements of voltage and current in the constant 15 impedance water transmission lines comprise the backbone of the electrical performance of the accelerator. These data give the module spread, voltage, power, and 10

energy for the entire accelerator. Figure 4 shows the Current (MA) voltage waveform from a single module of PBFA Z. The peak voltage is 2.5 MV with a 105-ns FWHM pulse. The 5 power shown is the 36 times the power of a single line. These data give a total forward-going energy for the 0 accelerator of ~ 4.5 MJ. 2350 2400 2450 2500 2550 2600 2650 2700 The peak stress measured in the water lines was Time (ns) 180 kV/cm. The water lines on PBFA Z can successfully operate at this stress level. Data show that breakdowns in Figure 7 The total stack current (solid line) is plotted with the water transmission lines is dominated by edge effects, the total MITL current for Shot 51. The stack flashes at joints, and gaps. We see no evidence of electrical ~ 2675 ns. breakdown or losses in the uniform field portion of the water transmission lines. 250

20 50 200

16 40 150 Power (TW) Power (TW) 12 30 100

8 20 50 Current (MA)

4 10 0 2400 2450 2500 2550 2600 Time (ns) 0 0 2350 2400 2450 2500 2550 2600 Figure 8 The total stack power (solid line) is plotted Time (ns) together with the x-ray power for Shot 51. Figure 6 The total current (solid line) and power at the 4.3 MITLs insulator stack measured on Shot 51. Data from MITL current monitors for Shot 51 show that there is 100% current transfer from the insulator stack to a 4.2 Insulator Stack location just radially inside the MITL current monitors (a Data show that the insulator stack is exposed to peak location just over 1 m from the load). The total current in voltages exceeding 3 MV. Our data show that at peak the MITLs is compared with the total insulator stack electric fields of ~ 100 kV/cm the insulator stacks never current in Fig. 7. Note that the current has exactly the flash. This voltage hold off value is at the extreme limit same shape as the insulator stack current at all times of classical insulator breakdown described in Refs. 1&4. except when the insulator stack flashes late in time. At In fact, even upon voltage reversal (after peak current) that time all of the magnetic flux is trapped in the MITLs when the voltage reaches as low as -1 MV the insulator and will only slowly LR decay away. MITL current data holds off the voltage for as long as 200 ns. Figure 5 from other shots with MITL current monitors just outside

1238 20 thereby generating x-ray powers of 200 TW and x-ray energies of 1.85 MJ. PBFA Z will be available for the next decade for a 15 variety of scientific uses. Experimenters can access unique regions of plasma density and temperature parameter space for basic science and defense applications. 10

Current (MA) ACKNOWLEDGEMENTS

5 The author would like to thank the invaluable assistance of a large number contributors, throughout government and industry, without whose pulse power expertise this 0 2350 2400 2450 2500 2550 2600 work would have been impossible. In particular, the Time (ns) authors acknowledge the significant contributions made by P. Corcoran, I. Smith at Pulse Sciences, Inc, San Figure 9 The total stack and MITL currents (dashed lines) Leandro, CA. for Shot 51 plotted together with the Screamer calculated values (solid lines). The Screamer load current is the bold *This work was supported by the United States line. Department of Energy under Contract DE-AC04- 94AL85000. the vacuum convolute (30 cm from the load) show no MITL current losses to that radial location. REFERENCES [1] R. B. Spielman, et al., Proc. Of the Tenth IEEE 4.4 Comparison of Data with Modeling Pulsed Power Conf., Albuquerque, NM 1995, pp. The final piece of data required to compare with circuit 396. [2] D. D. Bloomquist, et al., Proc. of the Sixth IEEE calculations is from the z-pinch load. We measure the Pulsed Power Conf., Arlington, VA edited by P. J. implosion time and have accurate knowledge of the initial Turchi and B. H. Bernstein (IEEE, New York, 1987), load mass and radial position. The load data, the measured p. 310. inductances, together with water transmission line, stack, [3] B. N. Turman, et al., Proc. Of the Fifth IEEE Pulsed 1 and MITL current and voltage data allows a detailed model Power Conf., Arlington, VA 1985, pp. 155.` B. N. Turman, et al., Proc. Of the Fifth IEEE Pulsed of the entire pulsed power system. We use the Sandia Power Conf., Arlington, VA 1985, pp. 155. Screamer code[5] with only one adjustable parameter, the [4] J. C. Martin, Fast Pulse Vacuum Flashover, effective loss impedance at the vacuum convolute, to SSWA/JCM/713/157 and AFWL High Voltage model PBFA-Z power flow. Notes, Note 2, 16 March 1971. Figure 8 shows the power and relative timing of the [5] M. L. Kiefer, K. L. Fugelso, K. W. Struve, M. M. Widner, SCREAMER, A Pulsed Power Design Tool, x radiation to the total insulator stack power. The timing 25 August 1995 (Sandia Internal Document). accuracy of the two signals is measured to be ~ 2 ns. Note that the z pinch provided a 4X power gain over the electrical power delivered to the insulator stack. Using this time information and knowing that the load dynamics places a severe constrain on the actual current driving the load we calculate all relevant voltages and currents in the system. Figure 9 plots the stack and MITL currents against Screamer calculated values. In addition, the Screamer prediction for the actual load current (not measured) is included. The agreements are excellent. Using the same modeling methodology with a large number of shots has given us confidence in this technique for extracting the actual load current for a wide range of load parameters.

5 CONCLUSIONS PBFA Z, representing the culmination of marx- driven water-pulse-line technology, is the world’s most powerful electrical accelerator. We are successfully operating at the 50-TW design point. We routinely couple MJ’s of electrical energy into imploding z-pinch loads

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